Full Idea
Psychophysical identity is a two-way street: if all mental properties are physical, then some physical properties are mental; but then all physical properties might be mental, or every property of everything might be both physical and mental.
Gist of Idea
Psychophysical identity implies the possibility of idealism or panpsychism
Source
David Lewis (New work for a theory of universals [1983], 'Min Mat')
Book Reference
'Properties', ed/tr. Mellor,D.H. /Oliver,A [OUP 1997], p.210
A Reaction
I suspect that this is the thought that has impressed Galen Strawson. The whole story seems to include the existence of 'mental properties' as a distinct category. This line of thought strikes me as a serious misunderstanding.